Role of Structural Defects and Oxygen Ion Migration in the Catalytic Activity of La,O,

Atomistic computer simulation techniques are used to investigate t h e role of key defect and transport properties of La203 that may be involved in the catalytic activity. The theoretical techniques are based upon efficient energy minimization procedures and Mott-Littleton methodology for accurate defect modelling. The interatomic potentials for La203 correctly reproduce the hexagonal structure. Anion Frenkel disorder is found to be the predominant intrinsic defect with negligible deviation from ideal stoichiometry, in accord with the known proper-ties of the pure material. The highest solubility, for a range of dopants, is calculated for Sr which would increase the oxygen vacancy concentration; this is consistent with experimental studies of the promoted catalyst. A low activation for 02- vacancy migration suggests fast diffusion through the bulk and to the surface. W e suggest that t h e catalytic activity is linked to an oxidation reaction in which oxygen-hole centres are created from the annihi-lation of oxygen vacancies by reactant gas-phase oxygen. Furthermore, calculated binding energies indicate the possible formation of stable (dopant*-) centres. Rare-earth-metal sesquioxides rank among the most active and selective catalysts for the oxidative coupling of methane to form higher hydrocarbon products (mainly ethane and ethene),'-'4 a process of considerable importance in utilizin